1. Field of the Invention
The present invention relates to an ion implanter having a
2. Discussion of the Related Art
FIGS. 4, 5 and 6 show a conventional ion planter which implants ions into a batch of semiconductor wafers 2 held on a wafer holding disk 1 by a centrifugal force. The peripheral portion 1a of the wafer holding disk 1 is conically curved. The inner surface of the peripheral portion 1a has a plurality of shallow recesses 1b which are wafer rests. When the disk 1 is rotated as the wafers 2 are in the wafer rests 1b, the wafers are pushed onto the wafer holding bottoms 1c of the rests so that the wafers are held on the disk. At the same time, an ion beam 3 is irradiated upon the wafer 2 in a direction shown in the drawings, to implant ions into the wafer. The beam 3 is irradiated upon the wafer 2 perpendicular to the conically-curved inner surface of the peripheral portion 1a of the disk 1. The disk is moved in a direction C as shown by one-dot chain lines in FIG. 4, so that the ions are implanted into the entire top surface of the wafer 2.
The wafer holding bottom 1c of each wafer rest 1b of the wafer holding disk 1 of the conventional ion implanter is a plane surface substantially circumscribed on the curve of the conically-curved inner surface of the peripheral portion 1a of the disk. Consequently, the wafer 2 is flatly held on the flat bottom 1c of the wafer rest 1b by the centrifugal force as the ion beam 3 is irradiated upon the wafer. For that reason, although the ion beam 3 strikes the top surface of the wafer 2 perpendicular to the surface at the center thereof, the angle of the beam to the surface deviates from the perpendicular off the center of the surface, as shown in FIG. 3. The deviation increases in proportion to the distance between the center of the top surface of the wafer 2 and the point of the irradiation of the ion beam 3 upon the surface, and becomes the maximum .delta. at both ends of the surface in the circumferential direction of the disk. This change in the angle of the irradiation of the ion beam 3 upon the wafer 2 results in altering the position, density, and distribution of the ions implanted in the wafer. If the diameter of the wafer 2 is small, the change in the angle is negligible. However, if the diameter of the wafer 2 is as large as 150 to 200 mm and/or the degree of integration of a semiconductor device made from the wafer rises, as in recent years, the implantation of the ions is required to be more accurate, namely, the deviation in the angle of the irradiation is not negligible and poses a problem in attaining uniformity of the quality of the ion-implanted wafer.